Passive monitoring of HF radio signals in determining multipath

Multipath considerations differ in HF communications to that of VHF mobile communications because of their differing modes of propagation. Whereas VHF waves propagate via line of sight and the resultant multipath occurring from scatterers is in the form of a delay spread, HF propagation operates predominantly in sky-waves refracting in ionised layers within the atmosphere. Multipath distortion in HF can occur both when signals interfere with each other causing amplitude fading, and also when signals of differing path lengths culminates in time delayed versions of the transmitted signal being received. This paper outlines methods of determining operating frequencies in HF communications (3-30 MHz) without the active transmission of probing signals and with the view of minimising multipath on a channel